The purpose of the present proposal is to study the properties of the gas exchange system at the levels of the lung, blood and tissue. Gas phase mechanisms affecting pulmonary gas exchange will be studied by evaluating stratified inhomogeneity, density dependent inspired gas distribution and reinspiration of series dead space gas. Dynamic responses to local alterations in ventilation and perfusion will be studied using 81Krypton. The saturation dependent interaction of other ligands with carbon monoxide-hemoglobin binding will be studied in whole blood. The interrelationships between perfusion distribution, hemoglobin-oxygen affinity and oxygen delivery will be studied in skeletal muscle.